Phenol resins, because of their excellent insulating properties, heat resistance and chemical resistance, have been used in a number of electronic materials as a constituent of a composite material containing various fillers. Particularly, as curing agents of various epoxy resins, many special phenol resins that are optimized in accordance with the required properties such as expansion coefficient, heat resistance, glass transition temperature (hereinafter, may be abbreviated as “Tg”), flexibility and water absorption have been studied and developed.
Resin compositions in which a phenol resin is used as a curing agent of an epoxy resin have been used in the cutting-edge electronic component applications. In recent years, performance enhancement and down-sizing of these electronic components have been rapidly advanced and, consequently, there is a problem as to how to dissipate the heat generated inside the components.
Resin compositions show excellent insulating properties and, also from the standpoints of their easiness of molding and processing, heat resistance and the like, resin compositions can be useful as heat dissipating materials. However, the thermal conductivities of ordinary resin compositions are lower by 1 to 3 figures than those of heat dissipating materials such as metals and ceramics. Therefore, in cutting-edge electronic components where an even higher thermal conductivity is required, there are cases where effective heat dissipation property cannot be attained.
As a practical method of using a resin composition, a method of preparing a composite material by mixing a resin composition with an inorganic filler having high thermal conductivity is known. However, in such a composite material in which an inorganic filler is mixed, since the thermal conductivity of the resin composition itself is low, in order to obtain effective thermal conductivity, it is required to mix a large amount of inorganic filler. The thermal conductivity of a resin composition is improved by mixing therein an inorganic filler having high thermal conductivity; however, this may cause a marked increase in the viscosity of the resulting composite material before molding, leading to extremely poor fluidity and filling properties. Therefore, an improvement in the thermal conductivity of a resin composition contained in a composite material is a very important problem.
For the above-described problems, there is an increasing number of reports where it is tried to improve the thermal conductivity of a resin composition from a structural perspective of an epoxy resin (see, for example, Japanese Patent No. 4118691, Japanese Patent Application Laid-Open (JP-A) No. 2008-13759, Japanese Patent No. 4595497 and Japanese Patent No. 4619770). As such reports, it has also been reported that, for example, by using an epoxy resin or the like having a so-called mesogen group of a biphenyl skeleton or the like in combination with a novolac-phenol resin derived from catechol, resorcinol or the like and increasing the orientation of the post-curing resin skeleton, the internal heat resistance can be reduced and the thermal conductivity of the resulting resin composition can thus be improved.
In WO 2007/086415, the present inventors reported that a novel phenol resin having a phenolic hydroxy group-containing xanthene derivative in the main chain can be obtained by performing a reaction using a dihydroxybenzene under special reaction conditions. Further, in JP-A No. 2007-262398, the present inventors also reported that a resin cured product showing high thermal conductivity can be obtained when this novel phenol resin is used as an epoxy resin curing agent.
Thus far, the present inventors discovered methods of producing a phenol resin having a xanthene derivative structure in the main chain. One example thereof is a method of producing a phenol resin having a xanthene derivative structure in the main chain in one step by allowing a naphthol or a dihydroxybenzene and an aldehyde to undergo a reaction in the presence of an acid catalyst under simple but special reaction conditions and thereby performing intramolecular dehydration and cyclization of the hydroxy groups between adjacent naphthol and phenol nuclei bound via 2,2′-methylene linkage (see, for example, WO 98/55523 and Japanese Patent No. 3375976). Particularly, in WO 2007/086415, it is described that a phenol resin having a phenolic hydroxy group-containing xanthene derivative structure in the main chain can be obtained.
Until now, it has been reported that a novolac phenol resin can be obtained by allowing a dihydroxybenzene such as catechol or resorcinol to react with formaldehyde or the like in the presence of an acid catalyst (see, for example, JP-A No. 2003-137950, JP-A No. 2005-281675 and JP-A No. 2001-55425). However, according to these documents, those novolac phenol resins that are obtained using catechol, resorcinol or the like have a hydroxy equivalent of about 60 and do not contain the phenolic hydroxy group-containing xanthene derivative structure described in WO 2007/086415.
Furthermore, in JP-A No. H10-147628, novolac phenol resins having a hydroxy equivalent of 75 or 77 that are obtained using a dihydroxybenzene are reported. However, since these phenol resins both use an aldehyde having an alkyl group, it is believed that the increase in the hydroxy equivalent is attributed to an effect of the substituent, not to the incorporation of a xanthene derivative structure.